The present invention relates to a composition of matter containing both ether carboxylate and sulfonate groups on the same molecule. The unique properties of the composition makes it suitable for many surfactant applications including but not limited to enhanced oil recovery (EOR), detergents, metal treating, agricultural formulations, mining, de-inking, paper processing, textile processing, coatings, adhesives, lubricants.
In the recovery of oil from subterranean reservoirs it usually is possible to recover approximately 15%-20% of the original oil in place by primary recovery. Secondary recovery methods such as well stimulation or water flooding are applied after the amount of oil recovered by primary recovery becomes uneconomical. Secondary recovery methods can recover approximately an additional 15%-30% of the original oil in place which leaves the reminder of the oil unrecoverable unless other means such as tertiary recovery processes are applied. These tertiary recovery methods include but are not limited to the use of miscible and immiscible gases and liquids, steam, foam, alkali, surfactants, and polymers.
It has been known that many factors including but not limited to the interfacial tension between the injection brine and the residual oil, the relative mobility of the injected brine, and the wettability characteristics of the rock surfaces comprising the reservoir are all important in determining the amount of oil recovered by tertiary recovery. Numerous studies have found that the addition of surfactants to the injection brine can alter the interfacial and wetting properties to help overcome the high capillary pressure and increase the oil recovery. In many cases the addition of a polymer along with the surfactant or immediately after the surfactant can increase the mobility ratio between the injected brine and oil thus further improving the sweep efficiency of the flood.
Because the injection brine composition varies, it is important to use the brine available at the injection site for the tertiary process in order to be economically feasible. It is important to have surfactants that are compatible with brines having wide ranges of total dissolved solids (TDS) and divalent cations such as those of calcium and magnesium. The problem with many of the presently used surfactants in tertiary oil recovery is that they are incompatible with the brines containing high TDS and divalent cations that are often found at the injection site. Costly water treatment processes or using an alternate fresh water source makes the tertiary recovery process economically unfeasible in many cases. Therefore it is important to have surfactants that are tolerant to the high TDS and divalent cations. It is also important that the surfactant be tolerant to the high temperatures encountered in some wells and to show limited adsorption on to the reservoir rock. Most surfactants cannot meet all these requirements and in many cases blends of several different types of surfactants are used to meet the specific requirements. When blends are used a strong possible of chromatographic separation exist as the surfactant blend propagates through the reservoir due to differential adsorption properties.
Anionic surfactants, especially sulfonates have been found to be very stable to high temperatures however they are not tolerant to brines of high salinities especially those containing small amounts of divalent cations. Ether carboxylates are very tolerant to high temperatures and high salinities but they fail to give the required low interfacial tensions required to recover oil. We have found a method of producing surfactants having both the properties of ether carboxylates and sulfonates on the same molecule enabling the recovery of oil from reservoirs having a wide range of salinities, hardness and temperatures.
Many examples of using mixtures of two or more surfactants to lower interfacial tension and recover residual oil can be found in the literature. U.S. Pat. No. 6,022,834 issued to Hsu et al discloses the use of mixtures of carboxylated anionic surfactants with sulfonated surfactants. U.S. Pat. No. 4,458,759 issued to Issacs teaches a composition comprising organic sulfonate surfactants such as sulfonate fatty acids having both weak and strong anionic functionality groups. These products are derived from fatty acids and as such cannot form stable ethers when reacted with ethylene or propylene oxides and therefore do not exhibit the thermal stability of the compounds described in the present invention. Processes and surfactants have been described in the literature using sulfonated oleic acid, for example, U.S. Pat. No. 3,575,883 to Foley. Besides being derived from acids to give unstable esters when alkoxylated, these employ conventional means of sulfonation and are limited to lower molecular weight products because of reduced sulfonation efficiency with high molecular weight products. The products of the present invention use a different sulfonation procedure and are not limited to low molecular weight products. Therefore highly alkoxylated products of molecular weights exceeding 1000 can be easily manufactured. The composition of the present invention is an ether carboxylate having an additional sulfonate group on the molecule. The ether carboxylate group has been shown to be very salt tolerant and thermally stable. The sulfonate group provides thermal stability as well as lowering the interfacial tension. The two negative charges on the same molecule help to lower adsorption unto reservoir rock that is usually negatively charged by electrostatic repulsion. This combination of an alkyl ether carboxylate and an alkyl sulfonate attached to an aromatic spacer disclosed in the present invention is unique and provides synergistic performance that has not been anticipated before.